1. Field of the Invention
The invention relates to a filter apparatus for the uniform filtering of plastic melts having a filter housing in the interior of which filter discs are arranged in parallel to one another along a central filter rod, through which filter discs flow the plastic melt to be filtered.
2. Prior Art
Known filter apparatuses generally have a variety of geometries, for example a filter pot geometry in which a single filter surface in cylindrical form forms the inner wall of the filter pot, or consist of a filter candle arrangement in which a number of filter candles are accommodated in a filter housing, or consist of a cylindrical filter housing fitted with filter discs.
EP-B 0 247 468 discloses a filter apparatus for plastic melts in which candle filters are arranged in the filter housing along concentric circles, which candle filters consist of a support body and a filter material and through which flows the plastic melt to be filtered. Each candle filter in the support body contains a displacement body which forms a gap of constant width with the inner wall of the support body. The packing density of the candle filters per unit volume decreases from the periphery towards the center of the filter housing. To decrease the flow volume, flow bodies are arranged between the candle filters.
DE-A-34 19 822 describes a filter apparatus of a screw extruder for filtering the plastic melt exiting from the screw extruder, which filter apparatus possesses a filter housing having an inlet and outlet bore hole having a reversing slide valve which can be slid therein transversely to the melt stream. At least one recess, which comprises an annular turned groove, in the reversing slide valve serves to receive a screen body. For uniform distribution of the melt stream in the filter apparatus and of the pressure forces acting on the filter surface, the recess has an annular shape, and a melt channel connected to the inlet bore hole is present in which the cylindrical screen body encloses the turned groove of the recess in a form-fitting manner. A through-bore hole running in the longitudinal axis of the reversing slide valve connects the inlet bore hole to the outlet bore hole in the filter housing. A displacement cone is built into the through-bore hole, as a result of which an annular collection channel is formed which widens in the direction of flow of the melt towards the outlet bore hole.
This filter apparatus is a filter pot, which generally has a single filter surface in cylindrical shape which is arranged in the housing wall of the filter apparatus. In the known filter apparatus, a uniform distribution of the melt stream in the filter is sought and of the pressure forces acting on the filter surface, and complete relief of the transverse forces acting on the guide surface of the reversing slide valve is sought. By means of the displacement cone in the melt channel, the resistance to flow of the melt is intended to be reduced to a minimum in the reversing slide valve, with cross-sectional conditions which are favorable to the flow.
From the engineering aspect it is desirable, at the installed filter area, to have a filter apparatus having volume as small as possible which has no dead zones or only very small dead zones. The filter area generally forms in a filter pot the majority of the pot periphery or of the housing wall, the external shape of the filter pot being similar to that of a cylindrical candle filter. If there are no dead zones, or only very small dead zones, in the filter apparatus, the residence time distribution of the individual particles is narrow and the melt degradation is minimal. The residence time distribution describes the proportion of the mass particles of the plastic melt which has left the filter apparatus after a particular time.
A precondition for keeping dead times as small as possible is complete flooding of the filter volume by the melt. This is best achieved by the velocity vector not changing, or changing only very slightly, within the filter pot either in magnitude or in direction. A structural design of a filter apparatus in which, for example, the melt flow is diverted by 90 to 180.degree., should therefore be avoided if possible.
Compact filter apparatuses which comply with the above-mentioned requirements may, according to the prior art, as described, for example, in the literature references "Filtrieren von Kunststoffschmelzen" Filtering of Plastic Melts! VDI-Verlag GmbH 1981, "Kunststoff 70", 1980 edition, Volume II, pages 753-758 and "Plastverarbeiter", Volume 33, 1982, No. 12, pages 1447-1454, best be implemented by the concentric arrangement of filter inserts in candle or disc form in filter housings.
In the case of filter apparatuses containing filter discs, in particular when they are arranged perpendicularly in the filter section, the apportioning of the part-streams and their throughputs over the individual filter discs, for example, are highly inhomogeneous. This leads to an elevated overall pressure drop and to a broadening of the residence time spectrum owing to the excessive residence times of the mass particles in the filter apparatus. Longer residence times and a broadening of the residence time spectrum cause a significant melt degradation, which is to be taken to mean that the mean molecular weight of the melt becomes lower owing to the longer action of elevated temperature on the melt.
The object of the invention is to improve a filter apparatus of the type described at the outset in such a manner that the throughput distribution of the melt over the individual filter discs is virtually constant, the residence time is made uniform and shortened, the residence time spectrum becomes constant and is kept narrow, the total pressure drop over the length of the filter apparatus is decreased and the melt degradation is kept very low.